This invention relates to ignitron devices, that is to say a discharge device comprising, within a sealed chamber, a pool of liquid metal (usually mercury) connected to a cathode terminal and an anode electrode provided above the surface of said liquid metal.
Unlike a mercury arc rectifier which is normally operated with high levels of current, an ignitron device is normally required to operate with relatively low levels of current. Frequently such devices are used in so-called "crow-bar" operations in which case, following high current crow-bar discharges, the power supply follow-through current can last up to 50 ms and is usually at a level of several amps. At low currents (e.g. below 10 A) for long durations (e.g. above 5 ms) the mercury discharge arc in the ignitron device becomes unstable and moves in a random manner across the surface of the liquid metal pool. It is believed that collisions between this unstable arc and the envelope wall of the ignitron occur and cause a metallic arc to occur on the metal surface contaminating the ignitron and "de-ageing" of the device. "De-ageing" results in a reduction in hold-off voltage. It is also believed that stray magnetic non-axial fields from nearby power devices (transformers etc.) may aggravate this problem.